So far bovine embryos obtained by in vitro fertilization and/or in vivo fertilization have been cultured for several days in in vivo (oviducts of rabbits and sheep) or in vitro (co-culture in the presence of oviductal or cumulus cells) for their development.
When bovine embroys obtained by in vitro culture of in vitro fertilized oocytes were transported to the distant place where the embryo transfer of them was carried out, they were cryopreserved using glycerol and sucrose as cryoprotectans, stored and transported in liquid nitrogen. However, this method of transportation of embryos is nothing but a trial.
It is well known that bovine embryos do not develop beyond 8 cell stage (8 cell block) in in vitro culture. To overcome this 8 cell block embryos have been cultured in vivo or in vitro (co-culture with a monolayer of cells).
However, it is time and effort consuming to culture embryos in vivo, because embryos must be transferred to recipient animals and collected from them surgically. In addition, as for culturing embryos with supporting (oviduotal or cumulus) cells in vitro, establishment of those cells is also time and effort consuming. Furthermore, sometimes the supporting cells cover the surface of bovine embryos resulting in distortion of the embryos. Therefore, it is necessary to develop more efficient in vitro culture system for bovine embryos.
When bovine embryos are transported to the distant place after cryopreservation, the viability of them decreases sharply during freezing and thawing of them. In addition, before transfer of cryopreserved embryos, a special facility for treatment of the cryopreserved embryos and examination of the viability of the embryos after thawing are needed. Therefore, it is necessary to develope a handy and reliable method for the transportation of bovine embryos.
While we have been investigating the suitable culture and transportation methods for bovine embryos, we found out that the reduced state of embryos during culture and transportation are important for them to develop. And we concluded that low molecular thiol compounds are suitable to make intracellular conditions of embryos reduced state and completed a new method of culture or transportation of bovine embryos.